Abstract
Net-zero emissions pathways increasingly rely on carbon dioxide removal (CDR) to offset emissions from difficult-to-decarbonize sectors and shortfalls in near-term mitigation. CDR, however, is not a single technology. The available approaches differ in their resource demands, where they can be deployed, and in removal timing. Across three studies, this dissertation examines how these differences shape the regional removal potential and the tradeoffs of large-scale reliance on CDR.
The Global Change Analysis Model for the United States (GCAM-USA) was used to model six classes of CDR under four scenarios to reach net-zero CO₂ emissions by 2050 in the United States. Because the resources CDR depends on are distributed unevenly, deployment was regionally concentrated, removing 1 to 1.9 GtCO₂ yr⁻¹ nationally by mid-century. In the Full Portfolio scenario, direct air carbon capture and storage (DACCS) predominated at roughly 50% of removal, followed by bioenergy with carbon capture and storage (BECCS) at 25% and enhanced rock weathering (ERW) at 11.5%, with Texas and the Midwest leading owing to their agricultural land and geologic storage. Restricting individual approaches shifted pressure between energy and land systems and amplified supply-chain side effects, more so when fewer technologies were available.
The same portfolios were then evaluated for their climate, air quality, public health, and economic implications using a framework coupling GCAM-USA with the Finite-amplitude Impulse-Response (FaIR) climate model and the EPA Co-Benefits Risk Assessment (COBRA) air quality and health impact assessment model. A 'no U.S. climate action' baseline, in which the United States takes no economy-wide action while the rest of the world progresses toward net-zero, was compared against high and low reliance on CDR (High- and Low-CDR) scenarios that both reach net-zero CO₂ by 2050 but differ in removal scale. Both avoided roughly $2.5-5.8 trillion (USD2020) in climate damages, but the High-CDR pathway cost $11-13 trillion while the Low-CDR pathway cost $16-20 trillion owing to deeper near-term fossil-fuel reductions. Public health benefits reached $2.8-6.5 trillion under High-CDR and $3.5-8 trillion under Low-CDR, preventing roughly 12,600 additional premature deaths by mid-century in High-CDR compared to Low-CDR, while heavy reliance on CDR could itself generate $5-6 trillion in revenues, exemplifying tradeoffs among health, economy, and climate.
Finally, spatiotemporal CO₂ removal rates for ERW and ocean alkalinity enhancement (OAE), governed by soil temperature and pH, ocean chemistry, air-sea gas exchange, and the mineralogy and grain size considerations of added minerals to agricultural lands and exclusive economic zones (EEZs), were incorporated into MESSAGEix-GLOBIOM under a 600 GtCO₂ carbon budget from 2020 to 2100. Because geochemical CDR can lag the application of alkaline minerals by years to decades, these lags limited its impact before mid-century but allowed it to support net-zero targets thereafter. Timing assumptions altered the scale, CDR technology mix, and carbon prices of the mitigation pathways by 2100. Using only the 20% most suitable zones of the global sites for ERW and OAE could remove CO₂ at the scale of 9 GtCO₂ yr⁻¹ in 2100, a figure grounded in process-based removal kinetics.
Together, these studies reveal that the resource demands, regional distribution, tradeoffs, and spatiotemporal removal dynamics of CDR are central determinants of their real-world contribution to climate mitigation. The prospect of large-scale future CO₂ removal should not justify delaying near-term mitigation. Scaling CDR to gigaton removal has so far been slow. The actual removal from geochemical CDR lags by years to decades, and the residual fossil emissions that heavy reliance on CDR sustains carry direct air quality and public health costs.
